AR# 9414

12.1 Timing Closure - Suggestions for paths through TBUFs or other intermediate points

Description

I placed a timing constraint on a path, but the constraint has errors. What can I do to make this timing constraint pass?

Solution

Is this a valid path? Sometimes a path through a TBUF is not valid. You can create a new time spec to ignore this path with the following syntax: FROM FFS THRU <output of tbuf> TO FFS TIG; If this is a valid path from FFS through the TBUFs to FFS, then create a new time group. There are two ways to create a time spec to cover this path: (Comment = #) #Creating a New Group; INST flip_flop_names TNM = new_group; #Exclude valid FFS from FFS group: TIMEGRP new_group = FFS EXCEPT valid_ffs; #Time spec Ignore: TIMESPEC TS_TIG = FROM FFS THRU "output of tbuf" TO new_group TIG; For additional suggestions and recommendations, see the following Answer Records: - Suggestions for avoiding high fan-out signals, see (Xilinx Answer 9410). - Suggestions for state machine optimization, see (Xilinx Answer 9411). - Suggestions for long carry logic chains, see (Xilinx Answer 9412). - Suggestions for I/Os 3-state enable paths, see (Xilinx Answer 9413). - Suggestions for timing through irrelevant paths such as RESET or ".SR" pin, see (Xilinx Answer 9415). - Suggestions for using multi-cycle paths, such as a path through a ".CE" pin, see (Xilinx Answer 9416). - Suggestions for how to avoid having too many levels of logic, see (Xilinx Answer 9417). - Suggestions for timing constraints that miss their goals by 5% to 10%, see (Xilinx Answer 9418). - Suggestions for timing constraints that miss their goals by 10% to 15%, see (Xilinx Answer 9419).